This invention relates to digital communications systems and, more particularly, to the processing of signaling information (e.g., on-hook, off-hook) that is passed between central offices via digital channel banks.
A digital channel bank is used as a voice terminal for the digital transmission network -- i.e., it typically interfaces a conventional switching machine and a digital transmission facility. It samples the speech amplitude and the signaling state of each of a plurality (e.g., 24) of trunks (voice channels) in rotation, multiplexes the information on a time division basis, and transmits it directly to a digital line (T1) or, via one or more stages of digital multiplexing, to a higher speed digital line. At the receiving end, another channel bank restores the original transmission amplitudes and signaling states.
Stated somewhat differently, the role of digital channel banks is to reproduce the voice and signaling information so that from the point of view of the office circuits, they are interconnected as though by metallic wires. Signaling continuity is maintained as follows. The incoming signaling state of each trunk is detected at each channel unit of a digital channel bank. Periodically (e.g., every 1.5 milliseconds) a digital line bit carries this detected information to the receive terminal or channel bank, and thence to the receive channel unit thereof. As each incoming signaling bit is detected, it is held (e.g., in a flip-flop store) until the next bit comes along. Relays are operated by the output of this process to reproduce, at the receive office, the detected signaling states.
The foregoing is admittedly a very sketchy and generalized description of the purpose and function of digital channel banks. In the exchange plant, the D3 Channel Bank is in wide use today and is described in greater detail in the article "The D3 Channel Bank" by W. B. Gaunt and J. B. Evans, Jr., Bell Laboratories Record, August 1972, pages 229-233. For intertoll (long-distance) use, the D2 Channel Bank has been in use since 1970; it is described in the articles "D2 Channel Bank: System Aspects" by H. H. Henning and J. W. Pan, The Bell System Technical Journal, October 1972, pages 1641-1657, and "D2 Channel Bank: Per-Channel Equipment" by C.L. Maddox and D. K. Thovson, The Bell System Technical Journal, October 1972, pages 1659-1673. There are additional articles in this October 1972 issue dealing with the D2 Channel Bank, but these are, for present purposes, merely cumulative.
When a digital carrier failure (i.e., loss of frame) occurs, the involved trunks are busied out by the associated trunk processing or carrier group alarm circuits in the digital channel bank. Historically, the time delay between fault detection and trunk processing committal has been in the order of several hundred milliseconds; the modern objective for this interval is 2.50 seconds. The longer interval is an attempt to reduce the trunk processing operations on short disruptions. Trunk processing, of course, takes the digital carrier out of service. A dilemma arises in trying to deal with the trunk signaling states in the interval between fault detection and trunk processing. The ideal solution would be to immediately freeze all signaling states at the channel unit signaling receivers when a fault occurs, and hold these states until the system restores or until the 2.50 second elapses and the trunks are processed. With this ideal situation, established connections are not broken and false sender seizures do not occur. However, a difficulty arises from the fact that a fault is not detected at the instant it occurs and by the time the fault is detected, false signaling information has likely been introduced into some of the channel unit signaling receivers.